Reference [44] mentioned one of the problems in the use of ionic liquids in the recovery of rare earth elements (and metals in general) that is often neglected by authors. This problem is the same in nature as those of ionic liquids, since, during the extraction process, an ionic liquid would enter the solution in the form of ions, causing the loss of the reactant and increasing the production cost; moreover, this method can also be considered as non-environmentally friendly. At the time of writing and/or publication, the use of these ionic liquids as extractants for REEs was still only in the laboratory research stage.

The abovementioned situation is common in investigations devoted to the use of ILs in the recovery of metals. Practically, the recovery of all metals using ILs has been investigated over time, with recent efforts described in the literature, including the recovery of platinum group metals from spent automotive converters using Cyphos IL101 and IL104 ILs [45], the separation of cobalt–nickel mixtures with the IL formed by the reaction of primary amine Primene JMT and Versatic 10 [46], the extraction of gold(III) from scrap electronic devices using A327H⁺Cl⁻ IL [47], the recovery of rare earths from electronic scraps [48], the recovery of gold, silver, and precious metals from chloride media [49], and the recovery of plutonium from spent nuclear fuels using ILs [50]. Ionic liquids have been also proposed for the recovery of sulfide from fuels [51]. The abovementioned examples indicate that there is still room to improve the knowledge of IL usage for REEs and other metals recovery in large-scale operations, at larger processing times, and in operational conditions such as counter-current mixers-settlers.

The mentioned toxicity, in conjunction with their non-biodegradable properties, precluded the use of ILs as neoteric solvents with green characteristics.

ILs serve as the basis of deep eutectic solvents (DESs), which formed a new class of chemicals characterized as being formed by mixtures of components of a eutectic, with the resulting product presenting a melting point lower than the values presented by the pure components [52,53].

Often, DESs were considered as a subclass of ILs because both share a series of physicochemical properties: low melting points, density, viscosity, low vapor pressure, the absence of or high flash-point values, and chemical and thermal stability; moreover, their properties can be changed to fulfill the requirements of these extractants to be used in specific cases. The difference in the properties of DESs, with respect to those of ILs, is somewhat advantagous in compensating for the shortcomings of ILs; also, since DESs can be non-ionic, these properties are clearly different between these two types of chemicals.

They are obtained by the complexation of a compound that acted as a hydrogen bond acceptor (HBA), typically a quaternary ammonium salt, and either a metal salt or an organic compound that acted as a hydrogen bond donor (HBD). Owing to the large combination of HBAs and HBDs, they can be tuned for various uses and are thus considered designer solvents.

DESs follow the nomenclature Cation⁺X⁻zY, where Cation⁺ refers to a cation (e.g., ammonium, phosphonium, sulfonium) with X⁻ being a Lewis base serving as a counter ion (often halide anions) and Y being a Lewis or Brønsted acid with z molecules interacting with the anion. With the next general formulae, one can account for five types of DESs:

Type I: Cation⁺X⁻zMCl_x, where M = Sn, Fe, Zn, Al, Ga, or In. They are similar to metal halide/imidazolium salt systems.

Type II: Cation⁺X⁻zMCl_x·yH₂O, where M = Co, Cu, Cr, Ni, or Fe. This type of DES differentiates from I in that it contains hydrated metal halides, which together with their relatively low cost allow compounds with lower melting points.

Type III: Cation⁺X⁻zRL, where R is an organic radical and L = CONH₂, COOH, or OH. This type is the most widely known and applied. Very often, these DESs are used as precursors of choline chloride (ChCl) and HBDs such as amides, carboxylic acids, and alcohols, which make them easily tunable, cheap, and sometimes biodegradable.

Type IV: MCl_x + RL = MCl_x−1⁺RZ + MCl_x+1⁻, where M = Al or Zn and L = CONH₂ or OH. In their composition, inorganic cations, not forming alone with low-melting-point eutectics, are present. ZnCl₂ and HBDs, such as urea, acetamide, or different diols, are normally components of this type of DES.

Type V. This type of DES was developed from the strong interaction between the acidity difference of phenolic and aliphatic hydroxyl moieties. Clearly different from the other types, the interactions between HBAs and HBDs are non-ionic in nature and, thus, this type of chemical is formed only by molecular compounds, i.e., the case of the thymol–menthol system.

Recently, for these types of DESs, different new terms have been added to the nomenclature of these non-aqueous solvents. Natural Deep Eutectic Solvents (NADESs) is among them, which refers to type III compounds combining ChCl, such as HBA, with naturally occurring carboxylic acids, sugars, and aminoacids; also, water was added on some occasions as a third component. Apart from these ChCl-based NADESs, others can be prepared from binary or ternary mixtures of carbohydrates with themselves (e.g., D(−)-fructose:sucrose(1:1) or D-(+)-glucose:D-(−)-glucose:sucrose(1:1:1)) or organic acids with sugar alcohols (e.g., citric acid:D-sorbitol(1:1)), among many others not requiring the presence of ChCl. NADESs were developed to explore the enhancement of the solubility of some intracellular compounds (flavonoid rutin, starch, albumin, etc.) that presented limited solubility in water. Additionally, therapeutic Deep Eutectic Solvents (THEDESs) are another type of chemical, in which active pharmaceutical ingredients are present and show potential in products where solubility problems should be avoided. Mixtures of menthol and ibuprofen have been proposed to enhance topical delivery systems.

Several reviews about the use of these DESs on the recovery of metals from various sources have been recently published [54,55,56,57,58].

Reference [59] described the use of DESs (and also ILs) to improve the floating characteristics of some minerals, such as rare earth minerals, quartz, and quartz hematite, as well as carbonate asphalt. The amount of DESs used in this field is smaller if compared with traditional flotation agents.

One of the various uses of DESs is to provide suitable media for the synthesis of REE-bearing compounds. A DES prepared by mixing choline chloride (ChCl) and urea (1:2) was used as a synthesis medium, via the solvothermal method, of praseodymium vanadate (PrVO₄) nanoparticles [60]. These nanoparticles were used as a solely capable electrocatalyst for the detection of furaltadone (FLO). Despite its mutagenic and carcinogenic properties, this FLO is an antibiotic that is commonly used in the growth of aquatic organisms and livestock.

The extraction of lanthanides from nitrate media by a non-ideal solvent composed of decanoic acid (C₁₀OOH) and trioctylphosphine oxide (TOPO) was investigated [61]. The maximum distribution of all lanthanides (from La(III) to Lu(III) was observed for an aqueous acidity of 0.5–1 M nitric acid (Figure 1), and the extraction decreased with the increase in the organic acid concentration in the organic phase; thus, TOPO was the key extractant for efficient extraction. No stripping data were available in the published manuscript.

Deep eutectic solvents were investigated in the recovery of rare earth elements from coal fly ash [62]. Mixtures of choline chloride (ChCl) with lactic acid (LA) and ChCl with para-toluene sulphonic acid monohydrate (pTSA) were used in the investigation. The first results showed better leaching rates when the molar ratios of ChCl:LA and ChCl:pTSA were 1:2 and 1:1, respectively. Both systems showed REE recoveries in the 85–95% range, though the best results were generally obtained with the ChCl:pTSA mixture, indicating the higher acidity provided by pTSA, which contributed to the REEs’ dissolution process. Both DES systems had about 5–8% higher dissolution than single LA, pTSA, and ChCl systems, with this enhancement in the dissolution being even higher, about 35%, when comparing the use of these DES systems with the use of sulphuric acid or another mineral acid (HCl, HNO₃). After dissolution, the leached liquor was diluted at 50% with demineralized water and REEs were recovered by direct chemical precipitation with oxalic acid.

The leaching behavior of rare earth (yttrium) carbonate before and after mechanical activation in choline chloride–urea–malonic acid deep eutectic solvents was investigated [63]. Without mechanical activation, yttrium dissolution was about 49%, while the leaching efficiency of yttrium was increased to 85% when the activation time was 60 min, this increase being attributable to the decrease in the particle size and the increased contact area of the reaction when mechanical activation was used. The dissolution process was controlled via chemical reaction and diffusion in the solid product layer. No data were provided regarding what to do with the Y-bearing DES phase.

A strategy for the separation of yttrium from heavy rare earth elements (HREEs) based on ternary hydrophobic deep eutectic solvent (HDES) extraction was proposed [64]. A total of 44 HDESs were prepared with four carboxylic acids serving as hydrogen bond donors (HBDs), bis(2-ethylhexyl) amine (BEA) serving as the hydrogen bond acceptor (HBA), and 1-decanol (DL) serving as the third component of the mixture. The prepared HDESs had the advantages of simple preparation, no purification requirement, low viscosity, low water solubility, and low toxicity. Among the various investigated compounds, the DL:oleic acid (OA):BEA-based HDES with OA as the HBD presented the best extraction ability, higher saturation loading capacity, and better phase separation stability. The separation factors of the DL:OA:BEA (9:1:5) mixture for HREEs (Dy-Lu) and Y in an industrial Y-enriched solution were Dy/Y ≥ 3.05, Ho/Y ≥ 3.37, Er/Y ≥ 4.29, Tm/Y ≥ 6.00, Yb/Y ≥ 10.8, and Lu/Y ≥ 11.2. The extraction of these elements was performed via ion association and partial cation exchange reactions, which took place simultaneously. The loaded HDES can be stripped from the organic phase with 0.2 M sodium oxalate or water.

In order to improve recovery strategies, electric-field-assisted mining has arisen as a technique to extract species from soils using green electrolytes to help in the extraction of metals. Thus, reference [65] evaluated the influence of various types of biodegradable electrolytes, including the use of deep eutectic solvents, in the electromining process. The soil, sampled from the northern region of Brazil, contained cerium(IV), lanthanum(III), and neodymium(III), and the DES-based solutions were prepared by mixing choline chloride and (i) acetic acid, (ii) citric acid, and (iii) oxalic acid in a 1:2 (ChCl:acid) molar ratio. Applying an electric field of 1 V/cm, as was somewhat expected, the use of the various electrolytes resulted in different solubilities. The maximum efficiency using only acetic acid resulted in nearly 70% of cerium(IV) recovery; citric acid removed 63% of lanthanum (III) and oxalic acid extracted 22% of the same rare earth. The results revealed that the use of the abovementioned DES+ acid mixtures did not improve the recovery of these REEs (Table 5); this is attributable to the possible degradation of choline chloride and the increase in the pH in the cathodic region. There was no mention of the recovery of these valuable REEs or what to do with the resulting solutions.

Similarly to ionic liquids [31], DES can be used as a medium to prepare rare-earth-bearing compounds with further uses. A hydrothermal method to prepare perovskite-type potassium niobate (KNbO₃) through a deep eutectic solvent (DES), which can be further used as an electrode material for the determination of bisphenol A (BPA), was investigated [66]. This nitrate was prepared from thymol (C₁₀H₁₄O) and menthol (C₁₀H₂₀O) in a 1:1 ratio.

The cyclic voltammetry of pure CeO₂, La₂O₃, Nd₂O₃, and PrO₂/Pr₂O₃ in the deep eutectic solvent Ethaline (1:2 mixture of choline chloride and ethylene glycol) was investigated, and the electrochemical activity of these oxides was assessed [67]. The electro-dissolution of pure oxides and water-leached monazite, after high-temperature pretreatment of the mineral, was carried out in a 0.1 mol/L glucose solution in Ethaline and showed a preferential solubility of about 24% for pure Nd₂O₃ against pure CeO₂, La₂O₃, and PrO₂/Pr₂O₃, which were found to be insoluble. It was also demonstrated that the electrodissolution of the water-leached monazite was not possible because of the inert behavior of the Ce_1−xLn_xO_2−x/2 solid solutions. This compound was formed due to the presence of CeO₂ in the product resulting from the high-temperature pretreatment of monazite at low mineral:Na₂CO₃ ratios. Thus, the formation of cerium oxide must be avoided as much as possible. No information was provided regarding what to do with the resulting solutions.

A deep eutectic solvent composed of isostearic acid (HA) and TOPO diluted in toluene was investigated for the selective recovery of scandium from iron, yttrium, and aluminum [68]. The use of a single isostearic acid or TOPO solutions in toluene did not allow the separation of theses elements; however, the use of this DES solution improved the separation of scandium and yttrium for the other elements. Moreover, undiluted DES improved metal extraction, i.e., a mixture 2:1 (HA:TOPO) allowed for the selective separation of Sc(III) from Y(III) and Fe(III) at pH values near zero. Scandium was extracted via the formation of a Sc(HA)₂A₃(TOPO)₅ complex in the organic phase, releasing three protons to the raffinate. This rare earth can be stripped from loaded organic phases via the use of 2 M HCl or sulphuric acid solutions.